A fire sale in finance is an emergency sale of assets at unusually cheap prices, because the owner desperately needs cash.
Originally the term meant a sale at very low prices of damaged goods when a store had a fire. They would have a sign saying "Fire Sale: everything must go!"
In science fiction and the movies, a "fire sale" is a computerized attack on the government, the transportation network, and the economy by computer hackers. The logic is that terrorists have decided "everything must go!"